1. Technical Field
The present invention relates to fixing units, image forming apparatuses, and image forming systems.
2. Related Art
Image forming apparatuses such as laser beam printers are well known. An image forming apparatus is provided with a cover that is openable and closable, and a fixing unit that is for fixing a developer image formed on a medium onto that medium and that is attachable to and detachable from the image-forming-apparatus body. The fixing unit has a first roller and a second roller that can be pressed in contact against the first roller, and in a state in which the cover is closed, the fixing unit fixes the developer image formed on the medium, which is pinched between the first roller and the second roller that is pressed in contact against the first roller via the medium, onto that medium. In order to make the second roller be pressed in contact against the first roller with a stable press-contact force, the fixing unit is provided with a spring, which is an example of an “urging member” that can urge the second roller.
Further, the fixing unit is also provided with a grip that is used for holding the fixing unit when, for example, a user etc. attaches or detaches the fixing unit to or from the image-forming-apparatus body. The fixing unit also has a handle for locking the fixing unit to the image-forming-apparatus body when the user etc. attaches the fixing unit to the image-forming-apparatus body.
When the fixing unit gets jammed, it becomes necessary for the user etc. to open up the cover and perform troubleshooting. In order for the troubleshooting to be carried out properly, it is preferable that the press-contact of the second roller against the first roller be released.
With this aim, some image forming apparatuses are provided with a press-contact releasing member that operates together with the opening movement of the cover to release the press-contact. In such image forming apparatuses, when the user etc. opens the cover, the press-contact releasing member moves in conjunction with the opening movement of the cover so as to release the press-contact.
As described above, in the above-mentioned image forming apparatus, the fixing unit can be attached to and detached from the image-forming-apparatus body. Therefore, there are instances in which the user etc. detaches the fixing unit from the image-forming-apparatus body. In a state where the fixing unit is detached from the image-forming-apparatus body, it is preferable that the press-contact of the second roller against the first roller is released, from the standpoint of preventing warping of the first and second rollers, for example.
When the user etc. detaches the fixing unit from the image-forming-apparatus body, the user etc. will first open the cover and then take the fixing unit off. As described above, when the cover is opened, the press-contact releasing member moves together with the opening movement of the cover, thereby releasing the press-contact.
When, however, the fixing unit is once detached from the image-forming-apparatus body, the fixing unit will be moved to a location far away from the press-contact releasing member. Thus, the function of the press-contact releasing member of releasing the press-contact will no longer be achieved. As a result, the second roller may get pressed in contact against the first roller when the fixing unit is in a state detached from the image-forming-apparatus body. In view of the above, there has been a demand for an image forming apparatus etc. with which the press-contact of the second roller against the first roller can be reliably released even in a state where the fixing unit has been detached from the image-forming-apparatus body.
Further, when the user etc. attaches the fixing unit to the image-forming-apparatus body, the user etc. will carry out, as a series of operations, an operation of holding and carrying the fixing unit to a fixing-unit holding section provided in the image-forming-apparatus body and an operation of locking the fixing unit to the image-forming-apparatus body.
Therefore, in order to improve user convenience, it is preferable that the above-described series of operations be carried out smoothly. In other words, there is a demand for a fixing unit etc. that has a high level of convenience for users etc.
Moreover, when the user etc. attaches a unit, such as the fixing unit, to the image-forming-apparatus body, there is a possibility that the user etc. may make an error in operation and the unit may not be attached in a predetermined position (i.e., the correct position) of the image-forming-apparatus body. In such a case, various problems (such as malfunctioning or damaging of the unit due to the image forming apparatus operating without the unit being attached in the predetermined position) may arise. Therefore, it is necessary to let the user etc. know that the unit has not been attached properly when the unit is not attached in the predetermined position.
Further, when the unit has not been attached in the predetermined position, the user etc. will have to attach the unit again in the predetermined position (i.e., reattach the unit). In order to allow the user etc. to carry out this reattachment smoothly, it is preferable to let the user etc. know that the unit has not been attached properly immediately after the user etc. attaches the unit in a position other than the predetermined position (i.e., the incorrect position). Accordingly, there is a demand for an image forming apparatus etc. that can let users etc. know that the unit has not been attached properly at an early stage.
Furthermore, in cases where the medium onto which the developer image is to be fixed is, for example, an envelope, it is known that crimples may occur on the medium if the press-contact force of the second roller against the first roller is too large. To address this problem, a fixing unit would be particularly advantageous, in which the intensity of the press-contact force of the second roller against the first roller can be changed, that is, in which a low press-contact-force mode can be employed when fixing the developer image onto a medium in which crimples are prone to occur, such as an envelope, from the viewpoint of preventing crimples, and a high press-contact-force mode can be employed when fixing the developer image onto a medium such as plain paper from the viewpoint of fixing ability.
Existing fixing units are provided with only a single spring for urging the second roller, and achieves the change in the press-contact force by changing the amount of expansion (or the amount of contraction) of the spring between the high press-contact-force mode and the low press-contact-force mode using a cam, for example.
However, in cases of changing the press-contact force through the above-described measure, there is a problem that a desired press-contact force cannot be obtained, due to the fact that it is difficult to set the amount of expansion of the spring to a predetermined value with high precision. Further, there is another problem that it is not possible to achieve a sufficient difference in press-contact force between the high press-contact-force mode and the low press-contact-force mode, due to restrictions in the size of the cam etc. Therefore, existing fixing units are not capable of changing the intensity of the press-contact force appropriately. Accordingly, there is a demand for a fixing unit etc. in which the intensity of the press-contact force of the second roller against the first roller can be changed appropriately.